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Lecture 2

Psyc 2220- readings week 2.docx

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PSYC 2220
Richard Murray

Psyc 2220 : Readings week 2, Chapter 3 Acuity- the smallest spatial detail that can be resolved Herman Snellen –invented the letter chart. Distance at which a person can just identify the letters/distance at which a normal vision can identify the letters. Otto Schade showed people sine wave gratings with different spatial frequencies and had them adjust the contrast of the gratings until they could just be detected. -Fergus Campbell and Dan Green, demonstrated that the human contrast sensitivity function (CSF) is shaped like an upside-down U, as shown in figure 3.7 Sine wave grating – a grating with sinusoidal luminance profile. Retinal Ganglion Cells and Stripes: -ganglion cells respond best when spatial frequency is just right usually in the medium frequency. -Cugell and Robson discovered that in addition to these cells responding to gratings of just the right size, they discovered that responses depend on the phase of the grating-its position in the receptive field. -when the grating has a light bar filling the receptive-field center and dark bars filling the surround, this causes the on-center cell to respond vigourously in turn increasing its firing rate. The Lateral Geniculate Nucleus -axons of retinal ganglion cells synapse in the two LGNs, one in each cerebral hemisphere act as a relay station on the way from retina to the cortex. -Magnocellular layer is bigger in size than the Parvocellular layer, and also the M pathways respond to large, fast-moving objects, and the parvocellular pathway is responsible for processing details of stationary targets. -between M and P layers we find the layers consisting of Koniocellular cells. Striate Cortex -consists of 6 major layers. A major and complex transformation of visual information takes place in the striate cortex. Topography of the Human Cortex -high resolution requires a great number of resources. To see the entire visual field with such high resolution, we might need brain too large to fit in our heads. That’s why we have high resultion in the center and low resolution in the periphery. -a major problem in the periphery is known as visual crowding-the deleterious effect of clutter on peripheral object recognition. Receptive Fields in Striate Cortex Hubel and Wiesel discovered that the receptive fields of striate cortex neurons are not circular, as they are in the retina and LGN. Rather they are elongated. As a result, they respond much more vigorously to bars, lines, edges, and grating than to round spots of light. -Also another property of the receptive fields of striate cortex neurons was that an individual neuron will not respond equivalently to just any old stripe in its receptive field. It responds best when the line or edge is just at the right orientation. This is called Orientation Tuning. Other cells in the striate cortex fire at different and all possible orientations. -So how are the circular receptive fields in the LGN transformed into elongated lines in the striate cortex? – the concentric LGN cells that feed into the cortical cell are all in a row. Other Receptive Field Properties Ocular dominance: property of the receptive fields of striate cortex neurons by which they demonstrate a preference, responding somewhat more rapidly when a stimulus is presented in one eye that when it is presented in the other. Simple and Complex Cells: -neurons are characters as simple cells when they have clearly
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